Abstract
The crystal structure of althausite was solved by direct methods and refined to a final R value of 0.022 for 1164 high-angle reflections (sin θ/λ > 0.45Å−1) with I > 2.5σ. The position of the hydrogen atom was found from a difference Fourier synthesis. The structure is orthorhombic Pnma with a = 8.258(2), b = 6.054(2), c = 14.383(5)Å. Magnesium atoms occur in both five- and six-fold coordination, and the coordination polyhedra are highly distorted. The Mg octahedra form chains along b by edge-sharing. Hydroxyl and fluorine occur in a largely ordered distribution among two different structural sites and occupy alternating positions along ‘channels’ parallel to b. Partial vacancy in the (OH,F) sites is confirmed, the population factor for the F site being 81 percent. The crystal-chemical formula of althausite is therefore Mg4(PO4)2(OH,O)(F,☐) with Z = 4.
The cleavages in althausite, {001} perfect and {101} distinct, occur along planes crossing relatively few bonds and leave the chains of Mg octahedra unbroken. Bond-strength calculations for althausite and wagnerite are presented and the OH content in wagnerite is discussed.
Preliminary results of hydrothermal syntheses indicate the existence of a series from F-wagnerite to OH,F-wagnerite and from OH-althausite to OH,F-althausite. Infrared spectra of fluorine-free althausite show a splitting of the O–H stretching frequency, proving that two distinct hydroxyl positions are present.
